In addition to the conventional speech transmission, modern mobile systems also provide subscribers with different data transmission properties. Mobile system services can be classified generally into teleservices and bearer services. A bearer service is a telecommunication service that provides the transmission of signals between user-network interfaces. For example modem services constitute bearer services. In a teleservice the network also provides services for the terminal equipment. Important teleservices in turn include speech, facsimile and videotex services.
The bearer services are usually classified into groups according to their characterizing features, e.g. asynchronous and synchronous bearer services. In an asynchronous bearer service, the transmitting and the receiving data terminal maintain their synchronization only for the transmission of each individual character. In a synchronous bearer service, the transmitting and the receiving data terminal are synchronized with each other during the entire data transmission. Within each such group there are a number of bearer services, such as a transparent service and a non-transparent service. In a transparent service, the data to be transmitted is non-structured, and transmission errors are only corrected by means of channel coding. In a non-transparent service, the data to be transmitted is structured into service data units, and transmission errors are corrected by using (in addition to channel coding) automatic retransmission requests. Further, each user data rate constitutes at present an individual bearer service. Therefore the number of different bearer services is great. For example in the Pan-European digital mobile system GSM (Global System for Mobile Communication), the data services for a single traffic channel presently comprise six different asynchronous bearer services for the rates of 300, 1200, 1200/75, 2400, 4800 and 9600 bit/s.
The present transparent services in the GSM system are based on a uniform end-to-end traffic channel, i.e. the data rate over each leg of the traffic channel and at the terminal interfaces is the same. According to this requirement, the GSM bearer services and the bearer services of the fixed network, such as the ISDN, must employ the same data rate. For example, if a transparent digital data call (e.g. UDI=unrestricted digital information) is set up between the GSM network and the ISDN, the user rate must be the same, e.g. 9600 bit/s, in both networks or the call fails.
Typically, the same mobile subscriber may have access to different teleservices and bearer services. He may use, for example, a speech service, a facsimile service and data services utilizing different types of bearer services. A call terminating at or originating from a mobile station may therefore require any one of these teleservices and bearer services, or a combination thereof, wherefore the correct service must be indicated to the mobile network. In calls originating from the mobile station, the call set-up signalling contains service data on the basis of which the mobile network is able to select the correct service. Also in calls originating from the ISDN (Integrated Services Digital Network), the signalling contains corresponding service data. If the call originates from, or is routed via, the public switched telephone network PSTN, however, no service data arrives at the mobile network. The solution to this problem is a multi-numbering scheme wherein a mobile subscriber has as many directory numbers as he has different services to which he wants to receive incoming calls.
The problem with the multinumbering scheme is the large number of bearer services. In order to alleviate this problem, the Applicant's co-pending PCT application PCT/FI96/00506 discloses so-called general bearer services: e.g. one asynchronous bearer service, one synchronous bearer service, one PAD Access bearer service and one Packet Access bearer service. This is achieved by providing the interworking between the mobile network and the fixed network with the following operations: negotiating a data rate between a mobile station and a mobile services switching centre, monitoring the data rate in the traffic channel leg between the mobile services switching centre and the fixed network, modifying the traffic channel mode of the mobile network, and decreasing the number of subchannels in high-rate HSCSD (high speed circuit switched data) services utilizing a multislot technique.
More precisely, in this procedure a mobile station MS and an interworking function IWF of the mobile services switching centre MSC first agree on a mutual maximum data rate to be used. The MSC IWF thereafter checks which data rate the fixed network traffic channel uses.
If the data rate in the fixed network is lower than the data rate negotiated by the MS and the IWF, the extra radio channel capacity is released and (in a transparent case) the mobile network traffic channel is modified to use the most efficient channel coding available in this case. In such a way, the data rate of the fixed network is adapted to the traffic channel rate of the mobile network by using the rate adaptation procedures of the mobile network so that a constant end-to-end data rate is ensured.
If the data rate in the fixed network is higher than the data rate negotiated by the MS and the IWF, flow control and buffering ensure that no data is lost in a non-transparent call. Transparent bearer services are not assumed to be supported in this case, since the traffic channel cannot support a constant end-to-end data rate. A terminal equipment in the fixed network transmits data to a fixed network traffic channel at a rate that is higher than the rate over the transparent traffic channel leg of the mobile network, and loss of data in the IWF is inevitable. In the other direction, the MS transmits data in a transparent traffic channel at a rate that is lower than the data rate over the traffic channel leg of the fixed network, and data integrity is lost in the IWF. For this reason, a call is released already during the call set-up if it is detected that the data rate over the fixed network leg is higher than the maximum data rate negotiated by the MS and the MSC for the transparent traffic channel. In other words, the mobile network does not support the use of transparent bearer services in such a situation.
The problem is that at present the ISDN does not usually provide, in practice, any kind of end-to-end signalling support that would carry parameters specific to the data call. In this situation, a calling ISDN terminal equipment can only use the default values of the parameters, which presently signifies probably the data rate of 19.2 kbit/s (and probably 38.4 kbit/s in future). On the other hand, for example the GSM network only supports the data rate of 9.6 kbit/s. This means that a transparent call is rejected after the IWF has detected the data rate employed over the fixed network traffic channel leg. If the IWF does not monitor the fixed network traffic channel, the call will be released in any case by the application or by the subscriber at either end since the transmission of data will fail.
If end-to-end signalling support is available and the calling ISDN terminal equipment signals a data rate (e.g. 19.2 kbit/s) that is higher than the data rate supported by the mobile network (e.g. 9.6 kbit/s), the call is released if it is transparent as a result of the negotiation between the MS and the MSC.